(1) It may come from receiving more than the physical is ready to assimilate. The cure is then a quiet rest in conscious immobility receiving the forces but not for any other purpose than the recuperation of the strength and energy.
(2) It may be due to the passivity taking the form of inertia—inertia brings the consciousness down towards the ordinary physical level which is soon fatigued and prone to tamas. The cure here is to get back into the true consciousness and rest there, not in inertia.
(3) It may be due to mere overstrain of the body—not giving it enough sleep or repose. The body is the support of the Yoga, but its energy is not inexhaustible and needs to be husbanded; it can be kept up by drawing on the universal vital Force but that reinforcement too has its limits. A certain moderation is needed even in the eagerness for progress—moderation, not indifference or indolence.
Ref : Letters on Yoga Vol. IV